Nuclear reactor vessels employed in the commercial generation of electrical power are of two types; the pressurized water type or the boiling water type. In either case, the reactor vessel utilizes a generally cylindrical metallic container having a base and a top flange welded thereto. The main cylinder portion itself usually comprises a series of lesser cylinders welded to each other. In addition, a plurality of circumferentially spaced nozzles extend through the main cylinder wall and are welded thereto. Thus, numerous welds are necessarily used in fabricating the reactor vessel, in mating the top flange to the main cylindrical body and in securing the inlet and outlet nozzles to the reactor vessel wall.
The reactor vessel, in use, is encased, in a thick concrete containment area. However, the structural integrity of the reactor vessel, the concrete containment notwithstanding, due to the operating environment is of critical importance.
The weld areas of the reactor vessel are, of course, inspected prior to its initial use. Such inspection is carried out with all portions of the vessel relatively accessible to an inspection device prior to its encasement in the concrete containment. However, in-service inspection of the reactor vessel welds is not only desirable, but is mandated under governmental regulations.
Under such regulations, it is required that the vessel weld areas be subjected to periodic volumetric examination whereby the structural integrity of the vessel is monitored. Due to the nature of an in-service inspection, the device designed to accomplish the specified weld examinations must be capable of successfully operating in an underwater and radioactive environment under remote control while maintaining a high degree of control over the placement and movement of the inspection sensors.
The operating constraints are further complicated by the variety of reactor vessel sizes to which the inspection device must be able to be accommodated. Furthermore, the inspection device must not only be compatible with the weld placements of the reactor vessels now in use, but must also be sufficiently versatile to adapt to inspection duty in future vessels. In addition, the inspection device must be arranged in its use to have only minimal impact with normal refueling and maintenance operations.
The use of ultrasonic transducers to inspect metal welds is known. One such system is described in the periodical Materials Evaluation, July 1970, Volume 28, No. 7, at pages 162-167. This article describes a transmitter-receiver type ultrasonic inspection system for use in the in-service inspection of nuclear reactor vessels. The positioning arrangement for the transducers uses a track which is mounted on the interior wall of the reactor vessel.
A method and apparatus for ultrasonic inspection of a pipe from within is disclosed in U.S. Pat. No. 3,584,504. In the apparatus disclosed therein, a transducer array is mounted on a carrier which is rotatable, by means of a central shaft of the apparatus, within the pipe.
In U.S. Pat. No.3,809,607, a nuclear reactor vessel in-service inspection device is detailed, which device is adapted to pemit remotely controlled and accurate positioning of a transducer array within a reactor vessel. This device comprises a positioning and support assembly consisting of a central body portion from which a plurality of radially directed support arms extend. The ends of the support arms are extended to and adapted for being seated on a predetermined portion of the reactor vessel to define a positional frame of reference for the inspection device relative to the reactor vessel itself. Repositioning and support assemblies are provided and include integral adjustment means which cooperate to permit the simultaneous variation of the extension of the support arms thereby allowing the inspection device to fit reactor vessels of differing diameters. A central column is connected to the positioning and support assemblies, which central column extends along the longitudinal axis thereof. One or more movable inspection assemblies are connected to the central column and include drive and position indicating means. Three specific inspection subassemblies include a flange scanner, a nozzle scanner and a vessel scanner. Each of these scanners employ multiprobe transmitter-receiver ultrasonic transducers to permit more accurate volumetric plotting of the integrity of the welds used in fabricating the reactor vessel.
Since the development of the above-identified inspection devices, the original inspection code has been amended to call for more reliable and more rigorous inspections. In addition, these prior art devices were unable to accurately measure or reach certain weld areas of the reactor vessel. Still other drawbacks in the prior art inspection devices were the reliability and speed of the actual inspection effort.
One particular problem which was not entirely solved by the above-described prior art devices was that of exactly positioning the inspection device with respect to the reactor vessel since knowledge of the position of the transducer array used in connection therewith is critical to accurately defining the size, orientation and location of a vessel or weld flaw. In addition, such exact positioning must be accomplished, that is, the inspection device must be fully seated in the reactor vessel at a known location, without damaging the ability of the top vessel flange to be sealed to its header.